Most of the 131 people Pfc. Amber Thill lists as friends on her MySpace.com page serve in the military. Some, like Thill's husband, are deployed to Iraq; others are serving in Afghanistan. MySpace, the 20-year-old Thill says, "is how most of us communicate."

This online link between troops serving overseas and their friends and families was interrupted Monday when the Defense Department announced that it had cut off access to MySpace, YouTube and 11 other popular file-sharing and networking Web sites on the Pentagon's 5 million computers and 15,000 networks.

The new policy, which military officials say is intended to reduce the amount of traffic snagging the Defense Department's overburdened worldwide network, comes on the heels of an Army regulation last month enforcing new, strict rules on soldier bloggers.

Although Defense Department officials said Monday's regulation is not intended to censor the ability of the troops to write home, the policy is emblematic of the military's effort to maintain a balance between operations security and privacy rights at a time when sharing information online is becoming commonplace, said Loren Thompson, a defense analyst at the Lexington Institute in Arlington, Va.

"U.S. military leaders are facing a dilemma in how to manage the access of war-fighters to the Internet and other information technologies," Thompson said. "On the one hand, they know such tools can bolster battlefield performance and morale. On the other hand, they know that enemies are adept at penetrating, manipulating or otherwise exploiting information networks."

The new regulation for the first time created a blanket ban on sites many troops use to share news, photos, video and audio with their family and friends. Military officials said they blocked the Web sites because they took up too much bandwidth.

"Our concern is network availability," said Timothy Madden, a spokesman for the Joint Task Force-Global Network Operations, an organization responsible for the Defense Department's network operations grid. "We're trying to make sure that we have enough capability to do what we need to do for (Defense Department) missions: ... war fighting, intelligence."

Maj. Bruce Mumford, communications officer for the 4th Brigade, 1st Infantry Division in Iraq, says the military will not spend more on expensive equipment to broaden the bandwidth and meet the demand.

Madden said U.S. troops are still allowed to access the sites on their personal computers and such nonmilitary networks as Internet cafes in Iraq run by private concerns. The troops also are allowed to send messages and photos by e-mail.

But another Pentagon spokesman, Col. Gary Keck, told the Associated Press that the regulation was also meant to "enhance and increase network security." He said that use of the video sites in particular was putting a strain on the network and also opening it to potential viruses or penetration by "phishing" attacks in which scam artists try to steal sensitive data by mimicking legitimate Web sites.

Military bloggers say the new rules are part of a concerted effort to suppress online publications by troops in the field. On many bases in Iraq and Afghanistan, Defense Department computers and networks are the only ones available to the troops.

"These blogs, these posts on MySpace were the last tenuous connection that an everyday American ... has to what the American military is, who the troops are," said retired paratrooper Matthew Burden, who runs the military blog www.blackfive.net. "This last tenuous connection will get severed by those regulations." It will definitely decrease the amount of communications back home," said Burden, who is based in Chicago.

"The vast majority of videos on YouTube posted by soldiers, their families and friends are personal messages, original songs, tributes and video letters," Julie Supan, YouTube spokeswoman, said in a statement. "We certainly don't want YouTube to be used to share sensitive security information or put anyone in harm's way. ... We look forward to meeting with the Department of Defense to learn more about their concerns."

The ban comes just days after U.S. forces in Iraq established a YouTube channel featuring combat footage "to give viewers around the world a 'boots on the ground' perspective of Operation Iraqi Freedom from those who are fighting it." That channel is meant to counter Iraqi insurgents and their supporters, who have been posting anti-American videos on YouTube for months.

But the contradiction inherent in the military's use of YouTube while prohibiting its access underscores "two competing camps in the military," said Noah Shachtman, who runs a national security blog for the San Francisco-based Wired magazine called the Danger Room at blog.wired.com/defense.

"There's one camp that is really eager to embrace all these digital media ... because they realize that ... a counterinsurgency information war is just as important as the one where stuff gets blown up," Shachtman said. "And then ... there are some old-school military guys that really see all these digital media literally as threats and think that the tiniest bits of the most seemingly innocuous information will somehow benefit the enemy."

Shachtman was, in part, referring to Army regulation AR-530-1, which went into effect last month. It bars military personnel from posting blogs without first clearing their content with superior officers.

That regulation recapped existing rules prohibiting service members from disclosing information that could jeopardize their mission or safety. Bloggers said it was aimed at bloggers such as Colby Buzzell, a former American machine gunner from San Francisco whose lucid, obscenity-riddled memoir about his year fighting in Iraq, "My War: Killing Time in Iraq," won the second annual Blooker Prize for the best book based on a blog.

Buzzell, 31, blogged for eight weeks during his deployment in 2004 before the Pentagon told him to stop, banned him from patrols and confined him to base, saying that he might have breached operational security with his writings.

Some military bloggers have decried the regulation, saying they have committed no major security breaches and that posting military blogs on MySpace and other Web sites helps rally public support for the troops and the war.

"Some of the best PR for the military has come from the military bloggers in the war zone," said Burden, who edited "The Blog of War," an anthology of frontline dispatches from Iraq and Afghanistan.

"There is no more honest, there's no more authentic, and there are no more gung-ho support voices than the soldiers themselves," Shachtman agreed. "To shut them up is really the equivalent of shooting yourself in the foot."

Thill, who is stationed at Fort Richardson, Alaska, and is scheduled to deploy to Iraq in July with her military police unit, said the limitations to MySpace.com will "affect us probably quite a bit, until we figure out (everyone's) e-mail addresses."